Re: [XEN PATCH v10 4/5] tools: Add new function to get gsi from dev

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.06.2024 12:23, Chen, Jiqian wrote:
> On 2024/6/20 15:43, Jan Beulich wrote:
>> On 20.06.2024 09:03, Chen, Jiqian wrote:
>>> On 2024/6/18 17:13, Jan Beulich wrote:
>>>> On 18.06.2024 10:10, Chen, Jiqian wrote:
>>>>> On 2024/6/17 23:10, Jan Beulich wrote:
>>>>>> On 17.06.2024 11:00, Jiqian Chen wrote:
>>>>>>> --- a/tools/libs/light/libxl_pci.c
>>>>>>> +++ b/tools/libs/light/libxl_pci.c
>>>>>>> @@ -1406,6 +1406,12 @@ static bool pci_supp_legacy_irq(void)
>>>>>>>  #endif
>>>>>>>  }
>>>>>>>  
>>>>>>> +#define PCI_DEVID(bus, devfn)\
>>>>>>> +            ((((uint16_t)(bus)) << 8) | ((devfn) & 0xff))
>>>>>>> +
>>>>>>> +#define PCI_SBDF(seg, bus, devfn) \
>>>>>>> +            ((((uint32_t)(seg)) << 16) | (PCI_DEVID(bus, devfn)))
>>>>>>
>>>>>> I'm not a maintainer of this file; if I were, I'd ask that for 
>>>>>> readability's
>>>>>> sake all excess parentheses be dropped from these.
>>>>> Isn't it a coding requirement to enclose each element in parentheses in 
>>>>> the macro definition?
>>>>> It seems other files also do this. See tools/libs/light/libxl_internal.h
>>>>
>>>> As said, I'm not a maintainer of this code. Yet while I'm aware that libxl
>>>> has its own CODING_STYLE, I can't spot anything towards excessive use of
>>>> parentheses there.
>>> So, which parentheses do you think are excessive use?
>>
>> #define PCI_DEVID(bus, devfn)\
>>             (((uint16_t)(bus) << 8) | ((devfn) & 0xff))
>>
>> #define PCI_SBDF(seg, bus, devfn) \
>>             (((uint32_t)(seg) << 16) | PCI_DEVID(bus, devfn))
> Thanks, will change in next version.
> 
>>
>>>>>>> @@ -1486,6 +1496,18 @@ static void pci_add_dm_done(libxl__egc *egc,
>>>>>>>          goto out_no_irq;
>>>>>>>      }
>>>>>>>      if ((fscanf(f, "%u", &irq) == 1) && irq) {
>>>>>>> +#ifdef CONFIG_X86
>>>>>>> +        sbdf = PCI_SBDF(pci->domain, pci->bus,
>>>>>>> +                        (PCI_DEVFN(pci->dev, pci->func)));
>>>>>>> +        gsi = xc_physdev_gsi_from_dev(ctx->xch, sbdf);
>>>>>>> +        /*
>>>>>>> +         * Old kernel version may not support this function,
>>>>>>
>>>>>> Just kernel?
>>>>> Yes, xc_physdev_gsi_from_dev depends on the function implemented on linux 
>>>>> kernel side.
>>>>
>>>> Okay, and when the kernel supports it but the underlying hypervisor doesn't
>>>> support what the kernel wants to use in order to fulfill the request, all
>>> I don't know what things you mentioned hypervisor doesn't support are,
>>> because xc_physdev_gsi_from_dev is to get the gsi of pcidev through sbdf 
>>> information,
>>> that relationship can be got only in dom0 instead of Xen hypervisor.
>>>
>>>> is fine? (See also below for what may be needed in the hypervisor, even if
>>> You mean xc_physdev_map_pirq needs gsi?
>>
>> I'd put it slightly differently: You arrange for that function to now take a
>> GSI when the caller is PVH. But yes, the function, when used with
>> MAP_PIRQ_TYPE_GSI, clearly expects a GSI as input (see also below).
>>
>>>> this IOCTL would be satisfied by the kernel without needing to interact 
>>>> with
>>>> the hypervisor.)
>>>>
>>>>>>> +         * so if fail, keep using irq; if success, use gsi
>>>>>>> +         */
>>>>>>> +        if (gsi > 0) {
>>>>>>> +            irq = gsi;
>>>>>>
>>>>>> I'm still puzzled by this, when by now I think we've sufficiently 
>>>>>> clarified
>>>>>> that IRQs and GSIs use two distinct numbering spaces.
>>>>>>
>>>>>> Also, as previously indicated, you call this for PV Dom0 as well. Aiui on
>>>>>> the assumption that it'll fail. What if we decide to make the 
>>>>>> functionality
>>>>>> available there, too (if only for informational purposes, or for
>>>>>> consistency)? Suddenly you're fallback logic wouldn't work anymore, and
>>>>>> you'd call ...
>>>>>>
>>>>>>> +        }
>>>>>>> +#endif
>>>>>>>          r = xc_physdev_map_pirq(ctx->xch, domid, irq, &irq);
>>>>>>
>>>>>> ... the function with a GSI when a pIRQ is meant. Imo, as suggested 
>>>>>> before,
>>>>>> you strictly want to avoid the call on PV Dom0.
>>>>>>
>>>>>> Also for PVH Dom0: I don't think I've seen changes to the hypercall
>>>>>> handling, yet. How can that be when GSI and IRQ aren't the same, and 
>>>>>> hence
>>>>>> incoming GSI would need translating to IRQ somewhere? I can once again 
>>>>>> only
>>>>>> assume all your testing was done with IRQs whose numbers happened to 
>>>>>> match
>>>>>> their GSI numbers. (The difference, imo, would also need calling out in 
>>>>>> the
>>>>>> public header, where the respective interface struct(s) is/are defined.)
>>>>> I feel like you missed out on many of the previous discussions.
>>>>> Without my changes, the original codes use irq (read from file 
>>>>> /sys/bus/pci/devices/<sbdf>/irq) to do xc_physdev_map_pirq,
>>>>> but xc_physdev_map_pirq require passing into gsi instead of irq, so we 
>>>>> need to use gsi whether dom0 is PV or PVH, so for the original codes, 
>>>>> they are wrong.
>>>>> Just because by chance, the irq value in the Linux kernel of pv dom0 is 
>>>>> equal to the gsi value, so there was no problem with the original pv 
>>>>> passthrough.
>>>>> But not when using PVH, so passthrough failed.
>>>>> With my changes, I got gsi through function xc_physdev_gsi_from_dev 
>>>>> firstly, and to be compatible with old kernel versions(if the ioctl
>>>>> IOCTL_PRIVCMD_GSI_FROM_DEV is not implemented), I still need to use the 
>>>>> irq number, so I need to check the result
>>>>> of gsi, if gsi > 0 means IOCTL_PRIVCMD_GSI_FROM_DEV is implemented I can 
>>>>> use the right one gsi, otherwise keep using wrong one irq. 
>>>>
>>>> I understand all of this, to a (I think) sufficient degree at least. Yet 
>>>> what
>>>> you're effectively proposing (without explicitly saying so) is that e.g.
>>>> struct physdev_map_pirq's pirq field suddenly may no longer hold a pIRQ
>>>> number, but (when the caller is PVH) a GSI. This may be a necessary 
>>>> adjustment
>>>> to make (simply because the caller may have no way to express things in 
>>>> pIRQ
>>>> terms), but then suitable adjustments to the handling of PHYSDEVOP_map_pirq
>>>> would be necessary. In fact that field is presently marked as "IN or OUT";
>>>> when re-purposed to take a GSI on input, it may end up being necessary to 
>>>> pass
>>>> back the pIRQ (in the subject domain's numbering space). Or alternatively 
>>>> it
>>>> may be necessary to add yet another sub-function so the GSI can be 
>>>> translated
>>>> to the corresponding pIRQ for the domain that's going to use the IRQ, for 
>>>> that
>>>> then to be passed into PHYSDEVOP_map_pirq.
>>> If I understood correctly, your concerns about this patch are two:
>>> First, when dom0 is PV, I should not use xc_physdev_gsi_from_dev to get gsi 
>>> to do xc_physdev_map_pirq, I should keep the original code that use irq.
>>
>> Yes.
> OK, I can change to do this.
> But I still have a concern:
> Although without my changes, passthrough can work now when dom0 is PV.
> And you also agree that: for xc_physdev_map_pirq, when use with 
> MAP_PIRQ_TYPE_GSI, it expects a GSI as input.
> Isn't it a wrong for PV dom0 to pass irq in? Why don't we use gsi as it 
> should be used, since we have a function to get gsi now?

Indeed this and ...

>>> Second, when dom0 is PVH, I get the gsi, but I should not pass gsi as the 
>>> fourth parameter of xc_physdev_map_pirq, I should create a new local 
>>> parameter pirq=-1, and pass it in.
>>
>> I think so, yes. You also may need to record the output value, so you can 
>> later
>> use it for unmap. xc_physdev_map_pirq() may also need adjusting, as right now
>> it wouldn't put a negative incoming *pirq into the .pirq field. 
> xc_physdev_map_pirq's logic is if we pass a negative in, it sets *pirq to 
> index(gsi).
> Is its logic right? If not how do we change it?

... this matches ...

>> I actually wonder if that's even correct right now, i.e. independent of your 
>> change.

... the remark here.

> Even without my changes, passthrough can work for PV dom0, not for PVH dom0.

In the common case. I fear no-one ever tried for a device with an IRQ that
has a source override specified in ACPI.

> According to the logic of hypercall PHYSDEVOP_map_pirq,
> if pirq is -1, it calls physdev_map_pirq-> allocate_and_map_gsi_pirq-> 
> allocate_pirq -> get_free_pirq to get pirq.
> If pirq is set to positive before calling hypercall, it set pirq to its own 
> value in allocate_pirq.

Which is what looks wrong to me. Question is what it was done this way in the
first place.

Jan